kofoid: development of limax. 93 



into a fresh-water environment, still retains the free-swimming larval 

 Btage characteristic of marine forms. It has acquired, however, the 

 "primitive segmentation cavity" found in the fresh-water Lamelli- 

 branchs, but not definitely known to be present in the marine forms. 



Lankester ('"4) does not refer to the cleavage cavity of Pisidium, nor 

 does he figure it except in comparatively late stages of development. 



Von Jhering ('7G) speaks of the three or four small cells in Cyclas, 

 whose progeny grow around the solid mass of the two large cells, and 

 of the later appearance of a cavity in the centre of this mass. Ziegler 

 ('85) finds a cavity in the thirteen-cell stage of Cyclas, but indicates no 

 cavity in the two earlier stages that he figures. 



The latest, and by far the most important, contribution to our knowl- 

 edge, of the cleavage cavity is that of Stauffacher ('93) upon Cyclas 

 cornea. The formation of a "true" cleavage cavity takes place at the 

 thirteen-cell stage by the gradual elevation of the cap of ectoderm cells 

 from the macromere to which they had been closely applied, resulting 

 in the development of a sharply defined space between the macromere 

 and its derivatives. This cavity persists and increases in size until it 

 ultimately becomes the relatively very large cavity of the blastula stage. 

 In addition to this cavity, which he regards as persistent from the 

 thirteen-cell stage on, Stauffacher finds in the two-cell stage a structure 

 which he regards as similar to that observed by Flemming in Anodonta 

 and by Rabl in Unio. He describes it as a disproportionately large 

 space, entirely unstainable, in the smaller of the two spheres, exactly in 

 the region where they are in contact. The cavity is filled with a fluid 

 free from granules. The protoplasmic part of the cell, which forms the 

 peripheral layer and contains the nucleus, merges very gradually into 

 this fluid-filled space. On the side of the macromere this space is 

 sharply and definitely limited. It seems from his description that this 

 space is regarded by him as lying in the smaller cell, i. e. intracellular, 

 though he does not distinctly designate it as such. The interpretation of 

 this space and its later history are best given in his own words {loc. cit., 

 p. 211) : " Es fallt bei Cyclas nicht schwer, den unumstosslichen Beweis 

 zu erbringen dass der genannte helle Raum in der That nichts mit einer 

 Furchungshohle zu thun hat. Dieselbe kornerlose Partie namlich, die 

 wir auf dem zweizelligen Stadium antreffen, ist zwar auch auf dem drei- 

 zelligen Stadium noch vorhanden, aber schon bei der Bildung der vierten 

 Furchungskugel wird sie bedeutend reduziert und verschwindet schliess- 

 lich ganz. Dagegen entstehen im weiteren Verlauf der Entwickelung 

 zwischen der grossen Mutterzelle und ihren jeweiligen letzten Abstain- 



